Conductive rubber is typically made by mixing 15-75 parts sponge-like high structure carbon black, such as a product sold under the trademark XC-72 manufactured and sold by the Cabot Corporation, with 100 parts uncured rubber, and then curing the mixture. It is known that the conductive properties of the mixture degrades with mixing. The degradation is most rapid when mixing is done at high shear. This may be due to the relatively large sponge-like particles of carbon black breaking down into smaller particles and thus reducing the average length of unbroken conductive path through the rubber. It is, of course, known that the physical properties of rubber improve with mixing but, with conductive rubbers physical properties are sacrificed in favor of electrical properties.
Normal commercial practice has been to add the minimum amount of the high structure carbon black into the uncured rubber, then mix at the lowest possible shear for the minimum time to get reasonable dispersion of the carbon black through the mixture. In an attempt to improve the physical properties of the rubber, a method has been used of adding carbon black to a part of a batch, then mixing this part with the remainder of the batch at low shear, for example, in a roll mill. The resulting conductive rubber has great variability from batch to batch and from region to region within the same batch in its volume resistivity. In addition, the conductive rubber exhibits large changes in resistivity with flexure and strain and degraded physical properties.
Conductive rubbers are used in application where the unique combination of properties afforded by such materials are desired. For example, conductive rubber has been used in medical operating room wear and floor coverings to prevent the buildup of static electricity in the operating room personnel and equipment. In these applications, variability in electrical conductivity and other physical properties can be tolerated.
The normally wide variability in electrical conductivity is not acceptable in conductive rubber for antennas. In addition, the normally large and variable interface resistance between the rubber and external electrical circuits in unacceptable in antenna applications.